1. Field of the Invention
This invention relates generally to a null lens testing apparatus for measuring flaws in an optical element, and more specifically, to a multiple-wavelength null lens testing apparatus incorporating both refractive and diffractive optical elements for measuring flaws in an optical element.
2. Background of the Invention
A null lens is a lens or system of lenses which creates a wavefront having a desirable or predetermined shape, and being free from aberrations not associated with the desired wavefront. Null lenses used to measure flaws or defects with great precision during the fabrication of an optical element are known in the art. See for example, Optical Shop Testing, Chap. 14, John Wiley and Sons, (1978), herein incorporated by reference. The null lens generates a wavefront shape to match the surface shape of the optical element under test if it were perfect. These types of null lens testing apparatuses generally incorporate Offner type null lens or computer generated holography. Typically, a null lens will generate a spherical wavefront for measuring a spherical optical element. Not only can a null lens testing apparatus be used to measure flaws of spherical optical elements, but can also be used to measure conic or aspheric optical elements. Unfortunately, these latter types of null lens testing apparatus have been heretofore limited to a single wavelength. For wavelengths other than the design wavelength, the null lens suffers chromatic aberrations of all orders. Consequently, limitations on a single wavelength prevent the null lens from testing aspheric optical elements at a wide variety of fabrication stages of the optical element due to the limits of the types and sizes of flaws a particular wavelength can detect. A null lens designed at an infrared wavelength is generally used during early optical fabrication stages, and a visible wavelength null lens is generally used for intermediate and final stages of fabrication of the optical element. Therefore, a plurality of null lens testing devices must be used to test a single optical element through its fabrication stages.
One method of utilizing a single null lens to perform multi-wavelength null tests is the use of a reflective null lens, known to those skilled in the art. However, reflective null lenses suffer the drawbacks of large size, great expense and heavy weight so as to severely limit the use of a reflective null lens for most null testing. In addition, most reflective null lenses generally require a refractive field lens, which generally has the same function as the second lens of an Offner refractive null lens system. Therefore, the operating wavelength is substantially limited to a single wavelength.
It is a further method known in the art to have achromatic or multi-wavelength null lenses by incorporating complicated lens systems such as lens doublets or triplets. However, because of the multitude of lens elements it is very difficult to meet the alignment and fabrication tolerances to obtain the proper certification of a null lens to reach modern requirements of many optical elements. Because of this, achromatic null lenses of this type are generally only possible for null testing of optical elements having relatively mild aspherics requiring less stringent tolerances. In the case of strong aspheric optics, it is almost impossible to correct the chromatic aberrations of all orders with a complicated lens system. What is needed then is a simple null lens testing apparatus which is capable of high precision multiple wavelength testing of conic or high aspheric optical elements. It is an object of a preferred embodiment of this invention to provide such an apparatus.